Base Of Mixed Layer Research Articles

Characteristics of Submesoscale Compensated/Reinforced Fronts in the Northern Bay of Bengal

AbstractFronts in the Bay of Bengal (BoB) are active and can potentially impact the regional dynamics such as temperature variability, salinity distribution and oceanic circulation. Based on the high resolution model output (LLC4320), this study investigates the characteristics of submesoscale fronts in the northern BoB and associated compensation/reinforcement effects. At sea surface, horizontal gradients of salinity and density are remarkable in the northern BoB, and they are nearly 3 times larger than temperature gradients. As the depth deepens, temperature gradients increase and become comparable to salinity gradients, while density gradients decrease a lot due to the increasing effects of compensation at subsurface. Statistical results show the dominance of salinity‐controlled fronts over temperature‐controlled fronts, and compensated fronts over reinforced fronts. The surface cooling/heating results in significant temporal variation of compensation at surface, but this variation is limited at subsurface by the blocking of the mixed layer base. The submesoscale‐selective feature of compensation is much more pronounced at subsurface layer than surface layer. From statistical analysis and idealized numerical model, we found the slump of salinity‐controlled compensated fronts are important in generating temperature inversion and maintaining barrier layer. This study validates the compensation theories originating from observations, and further illustrates the importance of subsurface compensated fronts using spatially continuous, regionally extended and longer‐term model output. The subsurface‐intensified submesoscale‐selective compensation is proved for the first time in this study.

Upper Ocean Responses to Tropical Cyclone Mekunu (2018) in the Arabian Sea

Based on Argo observations and a coupled atmosphere–ocean–wave model, the upper ocean responses to the tropical cyclone (TC) Mekunu (2018) were investigated, and the role of a pre-existing cold eddy in modulating the temperature response to TC Mekunu was quantified by employing numerical experiments. With TC Mekunu’s passage, the mixed layer depth (MLD) on both sides of its track significantly deepened. Moreover, two cold patches (<26 °C) occurred, where the maximum cooling of the mixed layer temperature (MLT) reached 6.62 °C and 6.44 °C. Both the MLD and MLT changes exhibited a notable rightward bias. However, the changes in the mixed layer salinity (MLS) were more complex. At the early stage, the MLS on both sides of the track increased by approximately 0.5 psu. When TC Mekunu made landfall, the MLS change around the track was asymmetric. Significantly, a cold eddy pre-existed where the second cold patch emerged, and this eddy was intensified after TC Mekunu’s passage, with an average sea surface height reduction of approximately 2.7 cm. By employing the stand-alone ocean model, the numerical experimental results demonstrated that the pre-existing cold eddy enhanced TC-induced MLT cooling by an average of approximately 0.41 °C due to steeper temperature stratification at the base of mixed layer. Moreover, heat budget analysis indicated that the pre-existing cold eddy also enhanced subsurface temperature cooling mainly through zonal advection.

Assessing the effective settling of mineral particles in the ocean with application to ocean-based carbon-dioxide removal

Ocean alkalinity enhancement (OAE), a potential approach for atmospheric carbon dioxide removal (CDR), can involve introducing milled mineral particles into the ocean to promote carbon dioxide uptake. The effectiveness of this method relies on particles remaining in the ocean mixed layer while dissolution takes place, which depends on particle settling rates. Conventionally, particle settling rates are assessed using the Stokes settling velocity in stagnant conditions. However, recent numerical modeling reveals that in dynamic, stratified ocean environments, sediment vertical transport can be up to an order of magnitude faster than Stokes settling because of two types of fluid instabilities that can take place at the mixed layer base. Here, we estimate effective settling velocities in the presence of these instabilities and assess the implications for the efficacy of this particular OAE approach for CDR. The new effective settling rate estimates are sufficiently rapid that there is negligible particle dissolution before particles settle out of the mixed layer. This result is independent of initial particle size for the range of sizes considered here. Findings underscore the importance of considering ocean dynamics and stratification in assessing particle settling rates and provide valuable insights for optimizing OAE applications in diverse marine settings.

On the Arctic Amplification of surface warming in a conceptual climate model

Over the last century Earth’s surface temperatures have warmed by order 1 K as a global average, but with significant variation in latitude: there has been most surface warming at high Northern latitudes, around 3 times more than in low latitude regions (termed Arctic Amplification), while there has been least warming over the Southern Ocean. Many contributing processes have been suggested to explain this asymmetrical latitudinal warming pattern, but quantification of the contributing factors responsible remains elusive. Complex general circulation climate models can reproduce similar asymmetrical patterns of warming, but it can be difficult to interpret the contributing processes. Meanwhile, idealised conceptual energy balance climate models have been able to reproduce a general polar amplification of warming whose origins can be interpreted, but this warming is often symmetrical across both hemispheres and may not be responsible for the real-world pattern. Here, we use a conceptual Energy Balance Model, with imposed closures for initial horizontal diffusivity and cloudiness drawing upon observational constraints and including temperature-dependent diffusivity and a sub-surface ocean heat reservoir, to show that the magnitude of present-day Arctic Amplification may arise through relatively simple thermodynamic (Clausius–Clapeyron) and radiative (climate feedback) processes. The current asymmetry between hemispheric warming may arise due to the transient heat transport up through the base of the surface ocean mixed layer from the slow-responding deep ocean to the fast-responding surface ocean being dominated by upwelling in the Southern Ocean. It should be noted that the processes identified here are not a unique in offering a potential solution, and so significant, or dominant, roles for dynamical processes remain plausible explanations for Arctic Amplification.

A seasonal climatology of the upper ocean pycnocline

Climatologies of the mixed layer depth (MLD) have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base remains poorly characterized, though this transition layer is an ubiquitous feature of the ocean surface layer. Available hydrographic profiles provide near-global coverage of the world’s ocean and are used to build a seasonal climatology of UOP properties – intensity, depth, thickness – to characterize the spatial and seasonal variations of upper ocean stratification. The largest stratification values O(10−3s−2) are found in the intertropical band, where seasonal variations of the UOP are also very small. The deepest (> 200 m) and least stratified O(10−6s−2) UOPs are found in winter along the Antarctic Circumpolar Current (ACC) and at high latitudes of the North Atlantic. The UOP thickness has a median value of 23 m with limited seasonal and spatial variations; only a few regions have UOP thicknesses exceeding 35 m. The UOP properties allow the characterization of the upper ocean restratification that generally occurs in early spring and is generally associated with large variability. Depending on the region, this restratification may happen gradually as around the Rockall plateau or abruptly as in the Kuroshio Extension. The UOP is also likely to merge intermittently with the permanent pycnocline in winter. The upper edge of the UOP is eventually close to MLD estimates, except in a few notable regions such as in the Pacific Warm Pool where barrier layers are important, and during wintertime at high latitudes of the North Pacific.

Generation of the Internal Pycnocline in the Subpolar Southern Ocean by Wintertime Sea Ice Melting

AbstractThe ocean's internal pycnocline is a layer of elevated stratification that separates the well‐ventilated upper ocean from the more slowly renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high‐latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanisms generating the internal pycnocline at southern high latitudes through the analysis of a high‐resolution, realistic, global sea ice–ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with sea ice‐ocean interactions in two distinct ice‐covered regions, fringing the Antarctic continental slope and the winter sea‐ice edge. In both areas, winter‐persistent sea‐ice melt creates strong, salinity‐based stratification at the base of the winter mixed layer. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice‐ocean interactions in configuring the vertical structure of the Southern Ocean.

Assessing the Spatio-Temporal Features and Mechanisms of Symmetric Instability Activity Probability in the Central Part of the South China Sea Based on a Regional Ocean Model

Symmetric instability (SI) is credited with one of the important submesoscale instabilities. However, due to its small scales, it is challenging to capture using current observational measurements and ocean models. Estimates of SI activity are useful for assessing whether SI should be parameterized. Based on a high-resolution ocean model, we use a criterion to assess the spatio-temporal features of SI activity without directly solving SI in the Xisha–Zhongsha waters. An Ertel potential vorticity (PV) analysis is performed, and the negative PV injection and frontal tendency are calculated to analyze the generation mechanisms. The results show that the activity of SI is strongly seasonal. In comparison, SI is active in winter, but it is inactive in summer. In addition, it is mainly found within the ocean surface mixed layer (SML), and it almost disappears in the base of the surface mixed layer (BML). Moreover, the vertical component of the Ertel PV leads to the vertical spatial difference of SI activity, and both the vertical component of the Ertel PV and the sea surface buoyancy flux play an important role in the seasonality of SI activity. The stronger frontogenesis around the Xisha Islands partially accounts for the horizontal distribution difference of SI. This work implies that the parameterization of SI may have potential value in practical application in this region in winter due to the high probability of SI activity.

Similarity of the turbulent kinetic energy dissipation rate distribution in the upper mixed layer of the tropical Indian Ocean

Turbulence within the upper ocean mixed layer plays a key role in various physical, biological, and chemical processes. Between September and November 2011, a dataset of 570 vertical profiles of the turbulent kinetic energy (TKE) dissipation rate, as well as conventional hydrological and meteorological data, were collected in the upper layer of the tropical Indian Ocean. These data were used to statistically analyze the vertical distribution of the TKE dissipation rate in the mixed layer. The arithmetic-mean method made the statistical TKE dissipation rate profile more scattered than the median and geometric-mean methods. The statistical TKE dissipation rate were respectively scaled by the surface buoyancy flux and the TKE dissipation rate at the mixed-layer base. It was found that the TKE dissipation rate scaled by that at the mixed-layer base exhibited better similarity characteristics than that scaled by the surface buoyancy flux, whether the stability parameter D/|LMO| was greater or less than 10, indicating that the TKE dissipation rate at the mixed-layer base is a better characteristic scaling parameter for reflecting the intrinsic structure of the TKE dissipation rate in the mixed layer, where D and LMO are respectively the mixed-layer thickness and the Monin-Obukhov length scale. The parameterization of the TKE dissipation rate at the mixed-layer base on the shear-driven dissipation rate and the surface buoyancy flux was further explored. It was found that the TKE dissipation rate at the mixed-layer base could be well fitted by a linear combination of three terms: the wind-shear-driven dissipation rate, the surface buoyancy flux, and a simple nonlinear coupling term of these two .

Mixed layer eddies supply nutrients to enhance the spring phytoplankton bloom

Mixed layer eddies resulting from baroclinic instability of fronts convert horizontal buoyancy gradients into vertical stratification, shoaling the mixed layer. In light-limited regimes – high-latitudes – this process can initiate phytoplankton blooms prior to the springtime warming. The question is whether mixed layer eddies can enhance the spring bloom by delivering nutrients from beneath the mixed layer. We couple a submesoscale-resolving model (SUB) with a simple ecosystem model and examine the role of mixed layer eddies on the development of the spring bloom. We compare the SUB simulation to two coarser resolution (10 km) simulations, one that includes a mixed layer eddy parameterization (MLE) and another that prescribes the restratification from SUB and advects the biogeochemical tracers using geostrophic velocities (NVF). The MLE simulates restratification of the mixed layer and bloom onset, but the spring bloom has a deficit of 10–13% in the new production compared to SUB. The NVF has the same restratification as SUB, and with no vertical flux of nutrients, leads to a spring bloom with a 32–40% new production deficit compared to SUB. Submesoscale processes lead to exchange across the mixed layer base, which is not represented in coarse resolution model simulations, even with mixed layer eddy parameterizations. Our results show that nutrients supplied by mixed layer eddies are important to enhance the spring bloom.

Periodic boundary layer separation and lateral intrusions observed above a sloping lakebed

AbstractLateral intrusions flowing from the boundaries of lakes and oceans might influence basin‐scale distributions of heat, chemical solutes, sediments, and organisms. Here, observations of numerous intrusions from multiple locations in a small lake were used to examine the relation of intrusions to a lakewide internal seiche, and to estimate mean exchange between sloping bottom boundary layers (BBLs) and the lake interior. BBL waters were observed periodically separating from the bed and flowing laterally offshore. Boundary layer separation was initiated where strong downslope flows encountered upslope‐propagating temperature fronts. These separation events propagated upslope, coherent with the vertically propagating internal seiche, resembling previously reported cases of upslope‐propagating internal bores. Following separation, jets flowed offshore immediately above temperature fronts, transporting boundary layer water at least 61 m into the interior. Jet propagation could be traced upwards, from the bed at 9.4 m depth to the base of the surface mixed layer. Mean offshore transport was most intense within the metalimnion (4–7 m depth). Intruding intermediate‐temperature thermocline water was likely supplied both by river inputs and by boundary mixing of cold and warm waters. These findings suggest that periodic jets, resulting from seiche‐induced boundary layer separation, transported significant quantities of water from the boundary layer toward the stratified lake interior, with possible implications for distributions of sediments, solutes, and organisms.

The Barrier Layer Effect on the Heat and Freshwater Balance from Moored Observations in the Eastern Pacific Fresh Pool

Abstract Formation and evolution of barrier layers (BLs) and associated temperature inversions (TIs) were investigated using a 1-yr time series of oceanic and air–sea surface observations from three moorings deployed in the eastern Pacific fresh pool. BL thickness and TI amplitude showed a seasonality with maxima in boreal summer and autumn when BLs were persistently present. Mixed layer salinity (MLS) and mixed layer temperature (MLT) budgets were constructed to investigate the formation mechanism of BLs and TIs. The MLS budget showed that BLs were initially formed in response to horizontal advection of freshwater in boreal summer and then primarily maintained by precipitation. The MLT budget revealed that penetration of shortwave radiation through the mixed layer base is the dominant contributor to TI formation through subsurface warming. Geostrophic advection is a secondary contributor to TI formation through surface cooling. When the BL exists, the cooling effect from entrainment and the warming effect from detrainment are both significantly reduced. In addition, when the BL is associated with the presence of a TI, entrainment works to warm the mixed layer. The presence of BLs makes the shallower mixed layer more sensitive to surface heat and freshwater fluxes, acting to enhance the formation of TIs that increase the subsurface warming via shortwave penetration.

Nonlinearly interacting entrainment due to shear and convection in the surface ocean

Large-eddy simulations were performed to investigate the entrainment buoyancy flux at the mixed layer base due to nonlinearly interacting shear-driven turbulence (ST) and convective turbulence (CT). The fluxes due to pure ST and pure CT were first evaluated, and their scalings were derived. The entrainment flux due to coexisting ST and CT was then evaluated and compared to the scaling-based fluxes due to the pure turbulences. It was found that nonlinear effects reduce the entrainment flux by 30 % when the turbulent kinetic energy production rates of ST and CT are comparable. The mixing parameterization schemes used in ocean general circulation models (OGCMs) fail to accurately reproduce the mixing due to the pure turbulences and/or the nonlinear effects, and thus the mixed layer depth (MLD). Because analysis using global datasets suggests that nonlinear effects are large at the mid-latitudes, these results indicate that the nonlinear effects might be responsible for the deep MLD biases in OGCMs and that mixing parameterization schemes need to be improved to correctly represent ocean surface mixing due to shear and convection, as well as waves, in OGCMs.

A Role for the Ocean in the Winter Sea Ice Distribution North of Svalbard

AbstractThe ocean is suggested to play a major role in the ongoing winter decay of the sea ice cover in the western Eurasian Basin. Using a high‐resolution sea ice‐ocean model, we investigate the processes influencing the ice‐ocean interactions in winter in the waters north of Svalbard, with a particular focus on those contributing to sea ice melt events of large amplitude. These short term events, lasting 5–10 days, are associated with locally large melt rates mostly found along the pathway of the Atlantic Water. The sum of all these events over the simulation period is found to contribute 40% of the total winter melt. Episodes of strong surface winds, occasionally associated with enhanced velocity shear at the mixed layer base, can trigger enhanced entrainment of Atlantic Water through the relatively shallow upper thermocline in the Atlantic Water boundary current, leading to substantial ocean heat transfer to the sea ice. In some cases, strengthening of the boundary current also contributes to fueling the heat transfer to the ice. Another type of large melt event, not linked to increased ocean vertical heat flux but due to ice being advected over warm surface waters, is also identified, sometimes associated with episodes of ice close‐up. Sea ice budget calculations show that, overall, large melt events contribute significantly to the eastward retreat of the winter marginal ice zone on the upper slope east of Svalbard while episodes of northward advection of ice largely dominate the ice edge retreat over the shelf north of Svalbard.

Interactions Between Ocean and Successive Typhoons in the Kuroshio Region in 2018 in Atmosphere–Ocean Coupled Model Simulations

AbstractTyphoons decrease sea surface temperature (SST) along their wakes through upwelling of subsurface water, vertical mixing in the upper ocean, and heat release from the sea surface, and these cold wakes can influence subsequent typhoons. In this study, we investigated interactions between the upper ocean and typhoons in the North Pacific subtropical gyre with focuses on Kuroshio's response and feedback using atmosphere–ocean coupled model simulations. In late August and early September 2018, typhoons SOULIK, CIMARON, and JEBI passed through the northwestern subtropical gyre. During the passages of SOULIK and CIMARON, SST decreased along their paths due to vertical mixing except in the Kuroshio region. We quantitatively revealed that the Kuroshio stayed warm because the deep mixed layer along its path and small vertical temperature gradient around the mixed layer base, which are unfavorable conditions for cooling by vertical mixing, limited the cooling effects. After SOULIK and CIMARON had passed, SST recovered through horizontal Kuroshio heat transport and radiative heating. The possibility that the SST field after SOULIK and CIMARON passages influences JEBI was also discussed. Although impacts on JEBI intensity were not identified, it is implied that the turbulent heat flux (THF; sum of the sensible and latent heat fluxes) around JEBI was modulated by the SST field: heat release from the ocean was reduced in the region with decreased SST and enhanced over the sustained high SST of the Kuroshio. Furthermore, the large THF over the Kuroshio may have caused an increase of JEBI‐associated precipitation around Japan.

Decomposing oceanic temperature and salinity change using ocean carbon change

Abstract. As the planet warms due to the accumulation of anthropogenic CO2 in the atmosphere, the interaction of surface ocean carbonate chemistry and the radiative forcing of atmospheric CO2 leads to the global ocean sequestering heat and carbon in a ratio that is nearly constant in time. This ratio has been approximated as globally uniform, enabling the intimately linked patterns of ocean heat and carbon uptake to be derived. Patterns of ocean salinity also change as the Earth system warms due to hydrological cycle intensification and perturbations to air–sea freshwater fluxes. Local temperature and salinity change in the ocean may result from perturbed air–sea fluxes of heat and fresh water (excess temperature, salinity) or from reorganisation of the preindustrial temperature and salinity fields (redistributed temperature, salinity), which are largely due to circulation changes. Here, we present a novel method in which the redistribution of preindustrial carbon is diagnosed and the redistribution of temperature and salinity is estimated using only local spatial information. We demonstrate this technique in the NEMO ocean general circulation model (OGCM) coupled to the MEDUSA-2 biogeochemistry model under an RCP8.5 scenario over 1860–2099. The excess changes (difference between total and redistributed property changes) are thus calculated. We demonstrate that a global ratio between excess heat and temperature is largely appropriate regionally with key regional differences consistent with reduced efficiency in the transport of carbon through the mixed layer base at high latitudes. On centennial timescales, excess heat increases everywhere, with the North Atlantic being a key site of excess heat uptake over the 21st century, accounting for 25 % of the total. Excess salinity meanwhile increases in the Atlantic but is generally negative in other basins, consistent with increasing atmospheric transport of fresh water out of the Atlantic. In the North Atlantic, changes in the inventory of excess salinity are detectable in the late 19th century, whereas increases in the inventory of excess heat do not become significant until the early 21st century. This is consistent with previous studies which find salinification of the subtropical North Atlantic to be an early fingerprint of anthropogenic climate change. Over the full simulation, we also find the imprint of Atlantic meridional overturning circulation (AMOC) slowdown through significant redistribution of heat away from the North Atlantic and of salinity to the South Atlantic. Globally, temperature change at 2000 m is accounted for by both redistributed and excess heat, but for salinity the excess component accounts for the majority of changes at the surface and at depth. This indicates that the circulation variability contributes significantly less to changes in ocean salinity than to heat content. By the end of the simulation excess heat is the largest contribution to density change and steric sea level rise, while excess salinity greatly reduces spatial variability in steric sea level rise through density compensation of excess temperature patterns, particularly in the Atlantic. In the Atlantic, redistribution of the preindustrial heat and salinity fields also produces generally compensating changes in sea level, though this compensation is less clear elsewhere. The regional strength of excess heat and salinity signals grows through the model run in response to the evolving forcing. In addition, the regional strength of the redistributed temperature and salinity signals also grows, indicating increasing circulation variability or systematic circulation change on timescales of at least the model run.

Flushing the Lake Littoral Region: The Interaction of Differential Cooling and Mild Winds

AbstractThe interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross‐shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density‐driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy‐driven cross‐shore transport assume calm wind conditions, which are rarely observed in lakes and thereby restrict their applicability. Here, we examine how moderate winds (≲5 m s−1) affect this convective cross‐shore transport. We derive simple analytical solutions that we further test against realistic three‐dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross‐shore winds. We show cross‐shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross‐shore exchange in the downwind region. The analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit‐width discharges in the upwind and downwind littoral regions. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross‐shore discharges of similar magnitude. This range is defined by the non‐dimensional Monin‐Obukhov length scale, χMO: 0.1 ≲ χMO ≲ 0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in situ observations.

Phytoplankton biomass and photophysiology at the sub-Antarctic Prince Edward Islands ecosystem in the Southern Ocean

At the sub-Antarctic Prince Edward Islands (PEIs), a diverse ecosystem is supported by enhanced phytoplankton biomass levels, commonly referred to as the island mass effect. Phytoplankton responses to varied environmental conditions during austral summer (December 2008) and autumn (April–May 2013–2018) cruises were characterised by both surface and subsurface chlorophyll a (chla) maxima. While subsurface chla maxima were generally near the base of the upper mixed layer (z uml ) during the summer cruise, such maxima were located well below z uml during autumn cruises. Non-photochemical quenching resulted in severe underestimation of surface in situ chla. Substantially low F v /F m and σ PSII in the surface layers during December 2008 confirmed the photoinhibitory effects of high light exposure during the day and suggested damage to phytoplankton photosystem II reaction centres. Physiological measurements revealed that phytoplankton communities were probably not limited by nitrate or iron. However, some nutrient stress may have resulted from low surface silicates, which showed comparatively more daytime depletion during December 2008 than April–May 2013–2015. While the spatial distribution of in situ phytoplankton biomass during December 2008 was reminiscent of the island mass effect, congruent satellite observations surprisingly revealed that chla in the open ocean surrounding the PEIs was in fact much higher than chla on the island shelf. Our findings suggested that upwelling and retention, and consequent phytoplankton biomass accumulation, usually associated with a Taylor column structure at the PEIs may have been disrupted by enhanced offshore advection stemming from the interaction of mesoscale eddies and fronts with the PEI shelf. However, more detailed investigations are required to properly understand the spatial and temporal variations of phytoplankton biomass and their physiological responses to environmental fluctuations. • Surface and subsurface chlorophyll a maxima were observed in summer and autumn. • In December 2008, these maxima were located near the base of the upper mixed layer. • During autumn 2013–2018, such maxima were well below the upper mixed layer. • Several daytime profiles were compromised by non-photochemical quenching. • Photophysiological parameters confirmed effects of daytime high light exposure.

Increasing Winter Ocean‐to‐Ice Heat Flux in the Beaufort Gyre Region, Arctic Ocean Over 2006–2018

AbstractOcean‐to‐ice heat flux (OHF) is important in regulating the variability of sea ice mass balance. Using surface drifting buoy observations, we show that during winter in the Arctic Ocean's Beaufort Gyre region, OHF increased from 0.76 ± 0.05 W/m2 over 2006–2012 to 1.63 ± 0.08 W/m2 over 2013–2018. We find that this is a result of thinner and less‐compact sea ice that promotes enhanced winter ice growth, stronger ocean vertical convection, and subsurface heat entrainment. In contrast, Ekman upwelling declined over the study period, suggesting it had a secondary contribution to OHF changes. The enhanced ice growth creates a cooler, saltier, and deeper ocean surface mixed layer. In addition, the enhanced vertical temperature gradient near the mixed layer base in later years favors stronger entrainment of subsurface heat. OHF and its increase during 2006–2018 were not geographically uniform, with hot spots found in an upwelling region where ice was most seasonally variable.

Vertical fluxes conditioned on vorticity and strain reveal submesoscale ventilation

AbstractIt has been hypothesized that submesoscale flows play an important role in the vertical transport of climatically important tracers, due to their strong associated vertical velocities. However, the multi-scale, non-linear, and Lagrangian nature of transport makes it challenging to attribute proportions of the tracer fluxes to certain processes, scales, regions, or features. Here we show that criteria based on the surface vorticity and strain joint probability distribution function (JPDF) effectively decomposes the surface velocity field into distinguishable flow regions, and different flow features, like fronts or eddies, are contained in different flow regions. The JPDF has a distinct shape and approximately parses the flow into different scales, as stronger velocity gradients are usually associated with smaller scales. Conditioning the vertical tracer transport on the vorticity-strain JPDF can therefore help to attribute the transport to different types of flows and scales. Applied to a set of idealized Antarctic Circumpolar Current simulations that vary only in horizontal resolution, this diagnostic approach demonstrates that small-scale strain dominated regions that are generally associated with submesoscale fronts, despite their minuscule spatial footprint, play an outsized role in exchanging tracers across the mixed layer base and are an important contributor to the large-scale tracer budgets. Resolving these flows not only adds extra flux at the small scales, but also enhances the flux due to the larger-scale flows.

Enhanced subsurface mixing due to near-inertial waves: observation from Seychelles-Chagos Thermocline Ridge

The Seychelles-Chagos Thermocline Ridge (SCTR) is a key region which plays a major role in various regional climatic processes at all time scales. The vertical mixing at the base of mixed layer may be climatically important at this region, where a shallow thermocline and highly variable wind activity exists. In this study, we provide evidence of strong vertical mixing at the base of mixed layer and thermocline associated with near-inertial waves (NIWs) using the microstructure profiler time series data during boreal summer 2014. During the study, it was observed that the surface turbulent layer was mostly confined in the mixed layer with turbulent kinetic energy dissipation rate of the order of 10−6 Wkg−1. Elevated dissipation rate in the order of 10−7–10−6 Wkg−1 was also observed at the base of the mixed layer and thermocline (60–100 m) during the entire period of observation. The estimated vertical diffusive heat flux across the mixed layer base (100 Wm−2) was strong and a moderate downward flux (~ 30 Wm−2) was noticed in the thermocline region. The analysis showed that the strong NIW triggered by a strong wind event were responsible for the high dissipation rate at the base of mixed layer and at deeper depths. The higher-mode NIW create elevated vertical shear at the base the mixed layer, which leads to the enhanced mixing. The study also showed the possible role of internal tides on the subsurface mixing in the region. The NIW-induced mixing may have important role on the mixed layer heat balance in a climatically active SCTR.